Lex Luthor, Deathstroke, and a "league of their own"
The final Justice League post-credits scene reveals that Jesse Eisenberg's Lex Luthor has broken out of whatever prison Batman threw him in at the end of Batman v Superman. (Was it Arkham? Let's say it was Arkham.) Now he's cruising in a fancy boat. In a less fancy sub-boat comes Deathstroke, portrayed by Joe Manganiello, originally cast in the role so he could appear as a villain opposite Ben Affleck in an untitled, standalone Batman film (though changing circumstances with that movie, which has handed directing duties over from Affleck to Matt Reeves, have made the Bat-star's star status unclear). Lex suggests that bad guys like he and Deathstroke start a "league of their own," which isn't a baseball team of women, unfortunately.
There are a bunch of evil teams in the DC Comics Universe, but the one portrayed most often in other media is known as the Legion of Doom. The Legion wasn't created on the pages of the comics, instead devised as an adversary for Hanna Barbera's animated take of the Justice League, Challenge of the Super Friends. They appear fully formed, and it isn't until a later episode that it's explained Lex Luthor brought together the twelve most dangerous villains to fight the Super Friends. The Legion of Doom then got adapted into the comics and has made appearances in other media since. The only problem with the Legion of Doom, if we're going to get nitpicky, is that Deathstroke isn't a member of any of the most famous incarnations. Also, Luthor isn't pitching a "Legion of their own."
Which brings us to the Injustice League, which is as confusing as most comic book continuity stories can be. The first Injustice League showed up in a 1989 run of the Justice League International comic book, and included Cluemaster, Major Disaster, Clock King, Big Sir, Multi-Man, and the Mighty Bruce as card-carrying members. Justice League International was a jokey book, so the the Injustice League was purposefully derivative and always up to unsuccessful schemes. In 2000, the Injustice League history was changed through retroactive continuity so the 1989 team was actually a second, lesser team; the "original" Injustice League was 19 villains united under an intergalactic entity named "Agamemno." The bored alien possessed Lex Luthor and got him to assemble the team, which did include Deathstroke. Since 2000, the Injustice League has been replicated on the small screen and in the pages of the comics (including the popular reboot event New 52) where the incarnations have included Luthor and Deathstroke. Some even include The Joker as a founding member.
The Injustice League sounds like a much more likely direction, should Warner Bros decide to continue down the current Justice League continuity. If the company doesn't, well, we got a successful Easter egg. And Lex got... a cool boat.